In passenger aircraft, sidewall monuments, otherwise known as structures such as closets, video control stations, lavatories and the like, are provided between the aircraft cabin or passenger compartment and the sidewall of the aircraft. At periodic spacings along the aircraft fuselage, vent openings and return air grilles are provided in the sidewall to establish flow of air from the cabin, along an air flow path adjacent to the sidewall and to the lower portion or cargo hold of the aircraft in the event of a decompression event in the lower portion. While it is desirable to facilitate the rapid flow of air between the lower portion and the passenger compartment of an aircraft, it is also desirable that the passage of air between such compartments may be obstructed when necessary.